The invention relates to a container for adding light metal to an aluminium alloy in the liquid state.
In metallurgical processes it is known to add light metals, such as alkali metals or alkaline earth metals, to other metals or alloys. In the preparation of aluminium-silicon, for example, it is common practice to make additions of a few ppm of sodium to the alloy in the liquid state, in order to give a fibrous structure to the eutectic system which develops when crystallisation takes place through cooling, and thereby to give the product obtained better mechanical properties.
In cases where ingots are produced the addition may be made in the melting furnace in the form of metallic sodium, or in the casting process in the feeding chute by means of aluminiun wire filled with sodium. In cases where moulded articles are produced the addition is also made in the feeding furnace in the form of flux or metallic sodium.
However, the addition cannot be made under the conditions which are normally used when other elements are added.
Alkali metals and alkaline earth metals in fact generally have a lower density than aluminiun, so if they are simply poured into the melting bath of alloy they will float to the surface and will not mix in well, even if agitated. As these metals are also very hygroscopic and oxidise easily in air, they will react at the surface of the bath and be converted to the hydroxide and/or carbonate form.
Hence the effectiveness of said metals is reduced. In addition to this effect, the presence of the products of the reaction gives rise to porosity or heterogeneousness which may make the alloy obtained brittle.
The addition must therefore be prevented from reacting at the surface, and for this purpose it must be inserted and completely dissolved within the bath. The resolution of this problem must be linked with the problem of excluding air during the storage and preliminary handling of the addition.
Solutions have indeed already been proposed, such as the use of bells, inside which the light metal is placed and which are immersed in the bath so that the metal cannot rise direct to the surface and the oxidation rate is therefore limited. However, since the area of exchange between the light metal and the bath is relatively large, the addition disperses too rapidly, so part of it still goes to the surface where it is degraded, thus reducing effectiveness by about 50%.
Other solutions consequently followed, usually consisting of placing the addition in a hermetically sealed container of the same type as the metal of the bath.
U.S. Pat. No. 3,848,391 describes the use of an aluminium box containing sodium or lithium and equipped with a fitting cover, for example for treating an aluminium-silicon alloy. The problem of excluding air during storage and handling of the addition is solved under these conditions, but not that of complete dissolution in the bath.
Since the boxes have a lower density than the bath, they tend to float. In addition, since the temperature of the bath is relatively higher than the melting point of aluminium, the box dissolves rapidly and liberates its contents abruptly. As a result sodium or lithium rises to the surface and there is a consequent oxidation reaction and loss of effectiveness.